For a cell to survive, it must be able to respond rapidly to changes in its environment. Furthermore, for cells to reproduce and carry out other co-operative functions, they must be able to communicate efficiently with each other. Cells most frequently adapt to their environment and communicate with one another by means of chemical signals. An important feature of these signaling mechanisms is that in almost all cases a cell is able to detect a chemical signal without it being necessary for the chemical messenger itself to enter the cell. This permits the cell to maintain tight control of its internal milieu, thereby permitting the cell to respond to its environment without being destroyed by it.
These sensing functions are carried out by a variety of receptors, which are dispersed on the outer surface of the cell and function as molecular antennae. These receptors detect an incoming messenger and activate a signal pathway that ultimately regulates a cellar process such as secretion, contraction, metabolism or growth. The major barrier to the flow of information is the cell's cellular plasma membrane, where transduction mechanisms translate external signals into internal signals, which are then carried throughout the interior of the cell by "second messengers."
In molecular terms, the process depends on a series of proteins within the cellular plasma membrane, each of which transmits information by inducing a conformational change--an alteration in shape therefore, in function--in the protein next in line. At some point the information is assigned to small molecules or even to ions within the cell's cytoplasm, which serve as the above-mentioned second messengers, whose diffusion enables a signal to propagate rapidly throughout the cell.
The number of second messengers appears at present to be surprisingly small. To put it another way, the internal signal pathways in cells are remarkably universal, and have been phylogenetically preserved over millions of years of evolution. Yet the known messengers are capable of regulating a vast variety of physiological and biochemical processes. The discovery of the identity of particular second-messenger substances is proving, therefore, to be of fundamental importance for understanding how cellular growth and function are regulated.
Several major signal pathways are now known, but two seem to be of primary importance. One employs cyclic nucleotides as second-messengers. These cyclic nucleotides activate a number of proteins inside the cell, which then cause a specific cellular response. The other major pathway employs a combination of second messengers that includes calcium ions as well as two substances whose origin is remarkable: inositol 1, 4, 5 trisphosphate (IP.sub.3) and diacylglycerol (DG). These compounds are released from the plasma membrane itself, by enzymes that are activated by specific cellular membrane receptors. However, it should be noted that myo-inositol in its non-phosphorylated form first enters an organism through the organism's diet, but can then be recycled as described hereinbelow.
IP.sub.3 is formed by the following scheme. A receptor molecule on the surface of the cellular plasma membrane transmits information through the cellular plasma membrane and into the cell by means of a family of G proteins, which are cellular plasma membrane proteins that cannot be active unless they bind to guanosine triphosphate (GTP). The G proteins activate the so-called "amplifier" enzyme phospholipase C, which is on the inner surface of the cellular plasma membrane. Phospholipase C cleaves the cellular plasma membrane lipid phosphatidylinositol 4, 5-bisphosphate (PIP.sub.2) into DG and IP.sub.3. IP.sub.3 is a water-soluble molecule and, therefore, upon being released from the inner surface of the cellular plasma membrane, it rapidly diffuses into the cytoplasm. IP.sub.3 then releases calcium from internal compartments, which store high concentrations of calcium. The calcium released by IP.sub.3 in turn activates a large number of intracellular enzymes that orchestrate a complex set of responses that allow the cell to adapt to the original signal triggering the receptor that caused the release of IP.sub.3.
Quite fascinatingly, DG and IP.sub.3 are recycled. DG is recycled by a series of chemical reactions which constitute one component of the lipid cycle. IP.sub.3 is recycled by a series of reactions known as the phosphatidylinositol cycle. The two cycles converge at the point when inositol is chemically linked to DG. The DG-bound inositol is phosphorylated in a series of steps which ultimately results in the resynthesis of phosphatidylinositol bisphosphate.
In the first portion of the lipid cycle, DG is converted to phosphatidic acid, which in turn is converted to cytidine diphosphate diglyceride (CDP-DG), while in the first portion of the phosphatidylinositol cycle, IP.sub.3 is dephosphorylated to ultimately form myoinositol. It is believed that such dephosphorylation occurs stepwise; IP.sub.3 is converted to an inositol bearing only two phosphate groups (IP.sub.2), followed by the loss of an additional phosphate, resulting in IP.sub.1, which is then dephosphorylated to myo-inositol. Also, it has been shown that IP.sub.3 can also undergo an additional phosphorylation, thereby being converted to inositol 1, 3, 4, 5 tetrakisphosphate (IP.sub.4). This molecule is subsequently metabolized by successive removal of phosphate groups, as described above. It is believed that phosphatase enzymes catalyses each step of this process.
The lipid cycle and phosphatidylinositol cycle merge by the myo-inositol reacting with the CDP-DG to form phosphatidylinositol (PI) PI is phosphorylated to ultimately form PIP.sub.2. It is believed that such phosphorylation occurs stepwise; PI is converted to phosphatidyl myo-inositol 4-phosphate (PIP), which is converted to PIP.sub.2 ; the final step of both cycles. It is believed that a kinase enzyme catalyses each step of this process.
For an excellent review of IP.sub.3, its role as a second messenger and the phosphatidylinositol cycle see Berridge, M., et al. Inositol Triphosphate, a Novel Second Messenger in Cellular Signal Transduction, Nature, 312, 315-321 (1984) and Berridge, M. The Molecular Basis of Communication Within the Cell, Scientific American, 142-152 (October 1985), and James W. Putney, Jr. (Ed.), Phosphoinositide and Receptor Mechanisms, Alan R. Liss, Inc., New York, N.Y. 1986.